The biosynthesis of estrogen is initiated by the synthesis of
Androstenedione, 19-carbon steroid hormone, from
cholesterol. Then this compound is converted to estrogens
Estrone or Estradiol, either
immediately or through Testosterone, which may also be
derived from cholesterol.

Androstenedione undergoes a three-step A-ring
aromatization to Estrone catalyzed by monooxyganases:
aromatase Cytochrome P450, family 19, subfamily A, polypeptide 1
(CYP19) [7] and Cytochrome P450, family 11,
subfamily B, polypeptide 1 (CYP11B1) [8]. The
first intermediate reaction is the formation of
19-Hydroxyandrostenedione [9], which then is
converted to Androst-4-en-3,17,19-trione [7], [9] followed by oxidation to Estrone [7], [9].

Another pathway of Estrone biosynthesis is oxidation of
17-alpha-Estradiol by Estradiol 17a-dehydrogenase
(1.1.1.148) [10].

Aromatase CYP19 also catalyzes oxidation of
Testosterone to Estradiol
[7], [11]. The first step is formation of
19-Hydroxytestosterone [12], which in turn is
oxidized to 19-Oxotestosterone [11], and then to
Estradiol [11], [13], similar to
Androstenedione catabolism to
Estrone.

Another steroid substrate, which undergoes a
CYP19-catalyzed three-step aromatization, is
16alpha-Hydroxyandrostenedione [14], [15]. The final product of that oxidation is
16alpha-Hydroxyestrone.

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